1. Field of the Invention
The invention is directed to a stepped lens with a Fresnel surface structure produced by lithography and with a radially symmetric or elliptic-optical effect by superimposing radiation dose proportions or distributions of cylindrical lenses with cylinder axes which are angularly offset relative to one another.
The subject of the present invention is further directed to a process for the fabrication of stepped lenses in which a resist coating is structured by superimposed exposure of two radiation dose distributions of cylindrical lenses with angularly offset cylinder axes and by a subsequent development process in which a development front is stopped from progressing into the depth of the coating.
2. Description of the Related Art
Lens structures must have great accuracy laterally and in profile. Lithographic methods for fabricating lenses with limited dimensions have been known for a long time. In principle, lithographic processes working with light beams, electron beams, x-ray beams or ion beams are suitable for producing such profiles. However, in many cases the problem of prolonged processing times arises in writing methods. This is because it is necessary to break up the curved structure geometries into a large number of structure details which must be processed to represent the profile surface.
This circumstance can be illustrated with reference to the production of a refractive micro-lens array by electron beam lithography. The process upon which this example is based is known as "variable dose writing" and makes use of the fact that the rate at which an electron-sensitive resist dissolves in the developer bath can be determined beforehand by the electron dose introduced in the resist. Accordingly, it is possible to design the electron dose distribution in such a way that the desired surface profile is obtained by interrupting the development after a certain period of time. This procedure is illustrated schematically in FIG. 1.
In the case of radially symmetric lens curvature of the individual lens and a finite vertical graduation in the profile, ring-shaped regions of uniform electron doses must be processed. When the lens is bounded by squares for efficient arrangement in the array, the outer rings of equal electron dosage are not closed. Due to the resulting large amount of data, each individual lens is processed relatively slowly. Very rapid electron-beam exposure systems work with a variable shaped beam having a rectangular cross section. However, their potential exposure speed can only be utilized if the square electron probe parallel to the coordinate axes can be used over the greatest possible surface area. But this is impossible in the present example, since a great many small squares must be processed to approximate the circular ring. FIG. 2 is a schematic illustration of the breaking up of a circular ring into squares. Even when using a high-speed electron-beam exposure system, the resulting processing times are unacceptably long.
According to the German Offenlegungsschrift 17 72 567, it is known to generate different optical path lengths by means of suitable, periodically changing exposure and subsequent development and re-halogenation and/or tanning development of a photographic coating. Gratings or rasters of spherical lenses are formed by exposing two systems of lines lying vertically or perpendicularly relative to one another in a square arrangement. It is disadvantageous that only lens rasters with lenses of substantially identical design and low optical quality can be produced using this solution.